Gas Switch Research Articles

A Compact Modular 5 GW Pulse PFN-Marx Generator for Driving HPM Source

A compact and modular pulse forming network (PFN)-Marx generator with output parameters of 5 GW, 500 kV, and 30 Hz repetition is designed and constructed to produce intense electron beams for the purpose of high-power microwave (HPM) generation in the paper. The PFN-Marx is composed by 22 stages of PFN modules, and each module is formed by three mica capacitors (6 nF/50 kV) connected in parallel. Benefiting from the utilization of mica capacitors with high energy density and a mini-trigger source integrated into the magnetic transformer and the magnetic switch, the compactness of the PFN-Marx system is improved significantly. The structure of the PFN module, the gas switch unit, and the connection between PFN modules and switches are well designed for modular realization. Experimental results show that this generator can deliver electrical pulses with the pulse width of 100 ns and amplitude of 500 kV on a 59-ohm water load at a repetition rate of 30 Hz in burst mode. The PFN-Marx generator is fitted into a cuboid stainless steel case with the length of 80 cm. The ratio of storage energy to volume and the ratio of power to weight of the PFN-Marx generator are calculated to be 6.5 J/L and 90 MW/kg, respectively. Furthermore, utilizing the generator to drive the transit time oscillator (TTO) at a voltage level of 450 kV, a 100 MW microwave pulse with the pulse width of 20 ns is generated.

Study on the Mechanism of Microparticle-Initiated Breakdown in the Air Gap

The microparticle-initiated breakdown plays a key role in the failure of gas switches and has become an active area of investigation in gas discharge recently. The mechanism of the microparticle-initiated breakdown in a 2-mm air gap is studied by experiments and simulations in this article. The dynamics of the microparticle in the electric field is obtained, and four breakdown modes are acquired from the picture of the high-speed camera. The probabilities of breakdown for the 200-, 300-, and 500- $\mu \text{m}$ Cu microparticles under different voltages are presented, and the corresponding lowest breakdown voltages reduce to 65%, 56%, and 47% of the static-breakdown voltage. The relation between the breakdown voltage and position of the microparticle is calculated based on the Townsend theory. The simulated results agree well with the experimental results. The breakdown process and evolution of the plasma are also revealed with PIC simulations.

Theoretical and numerical study on breakdown mechanism of nitrogen spark switch

Compared with the two-electrode gas spark switch, the three-electrode gas spark switch has high controllability, low working voltage and small jitter, so the three-electrode gas spark switch is widely used in pulse power technology. The discharge of gas spark switch is high pressure gas discharge, which is characterized by high electron collision frequency (10<sup>12</sup> Hz), small mean free path (10<sup>–6</sup> m), short breakdown time (10<sup>–9</sup> s), and complex physical process (including the secondary electron emission, the generation of seed electrons, the space charge effect and various collision processes between electrons and nitrogen molecules, etc). At present, it is difficult to quantitatively describe the breakdown process of the three-electrode gas switch, and the detailed theoretical research is lacking. Therefore, the breakdown mechanism of atmospheric pressure nitrogen spark switch, including two-electrode and three-electrode, is studied theoretically and numerically in this paper. The purpose of this study is to compare the simulation results of the two different gas spark switches, and obtain the characteristics of stream breakdown in different gas spark switches. Firstly, the numerical simulation and theoretical analysis of two-electrode gas spark switch are carried out. According to theoretical and numerical calculation, it can be found that for the plate-plate two-electrode switch, the stream breakdown cannot be generated under low voltage (less than 6.3 kV), while under high voltage (more than 6.3 kV), first the anode-directed streamer is formed, and then the cathode-directed streamer is created. In addition, the simulation results show that the plasma generated by the trigger can effectively reduce the breakdown voltage. Finally, the three-electrode gas spark switch is studied theoretically and numerically. It can be seen that in the breakdown process of the three-electrode gas spark switch, the breakdown first occurs between the trigger and the insulator, and then this plasma channel expands to the anode and cathode, finally forming the arc channel between the anode and the cathode. Under the calculation conditions in this paper, if the cathode-trigger and the anode-trigger are required to break down simultaneously, the applied voltage between the cathode-trigger should be greater than 1.18 kV, while the applied voltage between the anode-trigger should be greater than 3 KV. When the field emission of the trigger is considered, the breakdown threshold can be significantly reduced.

A gas-insulated mega-ampere-class linear transformer driver with pluggable bricks.

This paper presents the design and test of a gas-insulated linear transformer driver (LTD) cavity aimed at the Z-pinch experimental device CZ-34. The LTD cavity has a diameter of 2290 mm and a height of 346 mm. It consists of 23 main bricks and 1 trigger brick. Each main brick is comprised of two 100 nF capacitors connected electrically in series with a field-distortion gas switch. The trigger brick is comprised of two 50 nF capacitors connected in series with a compact multi-gap gas switch. All bricks are placed in the cavity filled with compressed SF6 and are pluggable like drawers. The trigger pulse generated by the trigger brick passes through an azimuthal transmission line to the trigger ring and makes the main bricks discharge synchronously. The LTD cavity can deliver ∼1 MA current pulse with a rise time of 115 ns to 0.08 Ω liquid resistance load when the charging voltage is ±100 kV, which is in good agreement with the circuit simulation results. Experimental results demonstrate the successful application of using gas insulation and pluggable bricks. The technical feasibility of the charging configuration, triggering method, and isolation resistors is verified. There is little difference in output performance as return-current rods replaced the outside metal cylinder, which provides a new path for the design of LTD cavities in series.

High action gas switches.

High action, high voltage closing switches are the key components of pulsed power systems based on high energy capacitor banks, primarily used for high power lasers, electromagnetic accelerators, high pulsed magnetic field facilities, crushing materials, and electromagnetic compatibility tests. There are several options for closing switches, including ignitrons, vacuum switches, pseudo-spark switches, solid-state switches, and high pressure gas switches (spark gaps). Spark switches are currently the most used due to their relatively simple design, reliability, and ease of maintenance and repair. The main disadvantage of spark gaps is a limited lifetime, which is directly or indirectly related to the erosion of the electrodes. To prevent erosion of the electrodes, multichannel switches and switches with movement of the discharge channel were proposed. In this Review, both types of switches are considered.

Multichannel switching in a multigap gas switch at atmospheric pressure

The multichannel switching mode of a controlled multigap switch is studied. The switch is designed for capacitive energy storage, with a charging voltage of up to 100 kV and an energy output time of about 100 ns. The interval between the high-voltage and low-voltage electrodes of this switch is divided into seven series-connected gaps using ball electrodes, and six parallel rows of these ball electrodes allow for multichannel switching. An optical system based on collimators and photodiodes was used to determine the number of channels in the switch gaps. Statistics on the number of channels were collected and the probability of ignition of parallel spark channels was calculated for each of the switch gaps under different operating conditions. We show that both the electrical isolation between the channels and the rate of rise of the triggering pulse significantly affect the number of parallel ignited channels and their distribution over consecutive gaps. At a high triggering voltage pulse rise rate ($\ensuremath{\sim}800\text{ }\text{ }\mathrm{kV}/\ensuremath{\mu}\mathrm{s}$), a larger number of spark channels are ignited in the switch with electrical isolation between the channels and the characteristics of this switch are better. At a low triggering voltage pulse rise rate (less than $250\text{ }\text{ }\mathrm{kV}\text{ }\ensuremath{\mu}\mathrm{s}$), better characteristics are realized in the switch without electrical isolation between the channels.

Effects of metal fluoride/sulfide microparticles generated by consecutive high-pulse-power breakdowns on the insulating performance in compressed SF6

Spark gas switches (SGS) are widely used in various pulsed power applications where sulfur hexafluoride (SF6) is still dominant because of its excellent insulating performance. However, particle contamination generated by SF6 possesses some special properties that can greatly reduce the effectiveness of the gas switches. The objective of this paper is to study the effects of particles generated spontaneously by consecutive breakdowns of high-pulse-power SGS on the insulating performance in compressed SF6. A double electrode/double pulse method, coupled with laser scattering and laser shadow photography, is adopted to detect the particles and examine their specific roles in the breakdown process. Many large particles in SF6, of approximately 150 μm in diameter, are observed at about 80 milliseconds after a single breakdown. Furthermore, numerous particles gradually suspend and accumulate in the gap after consecutive breakdowns. Particles generated by SF6 can reach tens of micrometers in size. They have rough microtopography covered by abundant floccules, and contain rich electronegative elements: fluorine and sulfur. Moreover, particle-involved abnormal breakdowns in SF6 usually occur with nearly equal threshold and probability in spite of the increasing consecutive breakdowns, and conversely, the abnormal breakdowns in N2 appear more frequently and unsteadily with a high dispersibility. The analysis of field enhancement caused by these large metal-fluoride/sulfide particles shows that they can directly cause significant field emission (FE) due to their properties of surface-field enhancement and causticity. Such FE further triggers micro-discharge. Subsequently, this behavior eventually leads to abnormal breakdowns at a lower threshold.

Review of the research on electrode erosion of gas spark switch

Gas spark switch is the most commonly used key device in pulse power device. The difficult problem of electrode erosion, which affects the performance of gas spark switches, has always become a bottleneck restricting the development and application of gas switch. This article reviews the existing electrode erosion theory and experimental research results, introduces the basic mechanism and simulation model of electrode erosion, summarizes the factors affecting the switching electrode erosion and the research progress of erosion-resistant electrode materials, and finally discusses the problems faced by the erosion research and the direction of optimizing the erosion resistance of the electrode materials.

Char reactivity and kinetics based on the dynamic char structure during gasification by CO2

Coal char reactivity is the essential for gasifier design and operation. Char structure and reactivity are dynamic with reaction during gasification. To obtain the relationship between dynamic structure and reactivity, char samples at different carbon conversion were prepared at 1223 K by a rapid heating thermogravimetric analyzer (R-TGA). The ex-situ char reactivity was investigated in TGA between 1073 and 1223 K, and was correlated with the dynamic structure. Besides of XRD and SEM results, nano-computed tomography (nano-CT) revealed the morphology of carbon matrix and minerals during gasification. Results showed that reaction rate of in-situ char significantly increased firstly and then decreased, which was attributed to the gas switch step, the increase of graphitization degree and inhibition effect of minerals. In contrast, the reactivity of ex-situ char was initially controlled by chemical structure and later by the abundant catalytic activity of minerals. The reactivity decreased slightly and then increased with increasing carbon conversion. The unreaction core model (URCM) was more suitable to interpret the gasification kinetics based on the dynamic char structure and fitting coefficient. Meanwhile, the results by iso-conversional method also supported the validity of URCM, but also indicated that the activation energy of char gasification gradually increased during in-situ gasification.

A High-Current All-Solid-State Pulse Generator Based on Marx Structure

In recent years, semiconductor switches have gradually replaced traditional gas switches due to their long lifetime and smaller volume. However, the rated current of the semiconductor switches limits the applications of the all-solid-state pulsed power generators. Usually, multiple semiconductor switches or pulse generators are required in parallel to share a large current in the case of the enormous power applications. In this article, a new high-current all-solid-state pulsed power generator based on Marx generators connected in parallel was proposed. One group of semiconductor switches was used to control the charging of the Marx generators. The driving circuits can be simplified due to fewer switches. It can also improve the discharge frequency and the output voltage over a capacitive load. Six 4-stage Marx generators connected in parallel have been employed to verify the proposed topologies on a capacitive and a resistive load. From the experimental results, with 1.2 kV, 150 A, and 2.4 kV, 50 A, pulses have been got on an 8- $\Omega $ resistive load and a 440-pF capacitive load, respectively. The balanced output currents can reduce the output voltage overshoot and make the system more stable and reliable.

Effect of gas type on insulation recovery performance and repetitive breakdown stability of a corona-stabilized plasma closing switch

The corona-stabilized plasma closing switch, which utilizes the stabilization effect resulted from the movement and accumulation of corona-generated space charge to improve the repetitive performance, has a great potential in repetitive pulsed power systems. In this paper, the influence of gas type on the recovery performance and repetitive breakdown stability of a corona-stabilized plasma closing switch is investigated. The results are analyzed from the memory effect of residual plasma, the development and propagation characteristics of streamer corona, and the thermo-hydrodynamic process of discharge channel. The enhancement effect of residual plasma on corona stabilization of the subsequent breakdown in the repetitive operation of the gas gap with highly inhomogeneous electric field and strong electronegative gas is proposed. The development and propagation characteristics of streamer corona in different gases that can lead to significant differences in the corona stabilization effect of breakdown are clarified. It is also found that the gas with small molecular mass and higher gas velocity exhibits excellent insulation recovery in the gas switch with uniform fields, but displays poor repetitive performance in the corona-stabilized switch with a rod-plate electrode.

Experimental research on methods to improve lifetime of insulators in multi-stage gas switches

We report on research into methods to improve the lifetime of insulators in multi-stage gas switches exposed to microsecond kV pulses at 20 Hz repetition rate. Two sets of experiments are reported; first on a comparison of polyetheretherketone (PEEK) and polyimide (PI) insulation. The results shows that the lifetime of PI far exceeds that of PEEK, which is attributed to the softening temperature of PI being higher than that of PEEK. In the second experiment, two methods are used to enhance the lifetime of PI; the first is to gild the surface of PI insulators, and the second is to insert PI films between the insulators and the electrodes. Both methods significantly increase the PI lifetime, but the film method leads to a longer lifetime than the gilding method. Imaging the insulator surfaces at the microscopic level reveals that local regions are not smooth, which results in greater electron emission from the cathode. The gilding method lowers electron emission and ejection and the film method blocks electron emission and ejection and thus they enhance the lifetime of PI insulators in multi-stage gas switches.

Triggering of Pressurized Gas Switches With a Class I Laser

The use of lasers to initiate the closure of high-power gas switches was initially explored soon after the development of suitable high-power pulsed lasers in the 1960s. The low jitter, excellent triggering range, and galvanic isolation provided by laser triggering have been extensively exploited in the triggering of megavolt switches in large pulsed power machines. The fact that the lasers and optical systems used in these machines are very large, complex, and expensive and have led to the perception that these are unavoidable characteristics of laser triggered gas switches, leading to a reluctance among the directed energy and compact pulsed power community to consider their use in compact pulsed power systems. The Advanced Pulsed Energy, Ionization, and Discharge Center (APERIODIC) Research Group at The University of New Mexico (UNM), Albuquerque, NM, USA, has been investigating novel triggering technologies for compact pulsed power. One of the technologies under investigation is a system we have named the microintegrated laser switch (MILS) [1]. This system holds the potential to overcome the limitations of traditional laser triggering of gas switches in compact applications. As part of the preliminary work on developing the MILS concept, UNM has collaborated with MegaWatt Lasers, Hilton Head Island, SC, USA, to conduct laser triggering experiments using a commercial off-the-shelf (COTS) erbium-doped glass passively Q-switched microlaser. This 28 mm × 9 mm laser produces 4-ns pulses at 1535 nm with an average energy per pulse of 244 μJ. One of the principal advantages of this laser is that, due to the wavelength and energy, it is a class 1 laser and can be operated without the stringent controls necessary for the lasers typically used for laser triggering.

Absorption of 1535-nm Laser Pulses in Insulating Oils

The Applied Pulsed Energy, Ionization, and Discharge Center (APERIODIC) research group at The University of New Mexico, Albuquerque, NM, USA, has been investigating the use of low-energy diode-pumped solid-state (DPSS) passively q-switched lasers operating at a wavelength of 1535 nm to trigger gas switches. This wavelength was chosen due to the improved eye safety and ready supply of commercial microlasers at this wavelength. Laser triggering of switches in a pulsed power system often presents challenges in beam transport from the laser to the switch optics. In many cases, it is most convenient if the beam transport can be accomplished directly through the insulating medium of the device. It is also possible that this laser wavelength could be used to trigger an oil-insulated spark gap switch. Thus, the absorption of the laser wavelength in insulating oils is of interest. In this work, the absorption coefficient of the 1535-nm laser pulses has been measured for various common insulating oils. The results indicate that beam transport through oil at this wavelength will likely be impractical due to relatively high absorption. However, the absorption is low enough that the use of this wavelength to trigger an oil-insulated gap could be possible.

Investigation of discharge process for argon spark switch at atmospheric pressure

In this paper, a 2D fluid switch model for argon spark switch at atmospheric pressure was established and the detailed discharge process was numerically investigated. The distributions of physical quantities such as electric field strength, electron density, particle density, electron temperature and electron migration velocity at different time were obtained. It is shown that at the initial moment of the discharge process, the electrons move from the cathode to the anode under the action of the external electric field. As the electric field strength increases, the streamer is first formed at the anode and rapidly propagates to the cathode, which finally forms streamer breakdown. The electron temperature starts to rise slowly near the location of anode and cathode, then decreases near the anode and rapidly transfers to the cathode. In addition, the repeated pulse discharge experiment based on the gas switch model was preliminarily carried out. And the current waveform and amplitude obtained by numerical simulation are basically consistent with the experimental results. The results indicate that the present research might provide theoretical basis for the study on the discharge process of gas spark switch.

A 80 kV gas switch triggered by a 17 μJ fiber-optic laser.

A gas switch triggered by μJ laser pulses was developed. The switch employed a 10 mm-gap GaAs photoconductive semiconductor switch (PCSS) mounted in parallel with one of its two gaps. The dark current-voltage characteristic of the PCSS was obtained, and the gas switch characteristics at different bias voltages were experimentally investigated. The results indicate that the switch can be triggered reliably by using a 17 µJ laser pulse, and the jitter is less than 3 ns at the bias voltage of 80 kV and 60% of the self-breakdown voltage.

A multiple-approximation method to calibrate switch capacitive voltage dividers without standard probe in Tesla-type generators

When a capacitive voltage divider (CVD) in a component is calibrated, the introduction of a standard high-voltage (HV) probe usually affects the original field distribution of this component. Therefore, the voltage ratio of the CVD is affected, especially when the distance between the HV probe and the CVD is close. A multiple-approximation method to calibrate the CVD of gas switches in Tesla-type generator without a standard HV probe is being put forward. The basic idea is to calculate the theoretical switch voltage uth based on the equivalent circuit of this type of generators and to use uth to substitute the real voltage ur acquired by the standard HV probe to calculate the voltage ratio β for the CVD of a gas switch. Since the gas-gap resistance Rs of the switch is not zero, uth is not equal to ur, and β therefore deviates from the real value β0. To solve this question, two steps are implemented: first, β(d) at different gas-gap distances d is obtained; second, the true voltage ratio β(0) when d = 0 is extrapolated. The theoretical basis of this method was analyzed first. Then, it is verified by experiments. In the end, the uncertainty of this method is evaluated.

Laser-triggered gas switch with subnanosecond jitter and breakdown delay tunable over ∼0.1–10 ns governed by the spark gap ignition angle

We created a practical air-filled switch triggered by a ∼109 W picosecond laser beam with a slightly variable breakdown delay and low jitter. The switch relies on the spark gap with a coaxial geometry, which is ignited by a focused laser beam directed at a certain angle to the gap axis. The switch is integrated with a high-voltage cable generator operating at a constant negative voltage of up to 50 kV. We demonstrate that, by just varying the ignition angle of the spark gap, one can achieve variable breakdown delay tunable within ∼0.1–10 ns with ≲1 ns jitter. Empirical dependences of switching characteristics of the developed device on the ignition angle are obtained. We demonstrate that, combined with variation of the ignition beam energy and charging voltage, variation of the spark gap ignition angle provides superior control over the gap switching characteristics without complicating the switch design. The proposed approach to driving the switching characteristics appears highly promising for designing compact laser-triggered gas switches with variable temporal characteristics and achieving precise synchronization between high-voltage and measuring equipment.

Best choice of insulation gas medium for MMGS in fast linear transformer driver

Gas spark closing switch is one of the most important components for a fast linear transformer driver (FLTD). In this paper, we describe the investigation of insulation gas media for a multi-gap multi-channel gas spark closing switch. A six-gap gas switch with corona needles was tested in a typical FLTD brick with two capacitors of 40 nF and a load resistor of about 10 Ω. Corona discharge current, self-breakdown voltage distribution, and triggered breakdown performance were tested when the gas switch was filled with air, N2, CO2, SF6/N2, and C4F7N/N2. When C4F7N/N2 was applied, there was no abnormal breakdown with low voltage found in the whole test process; the trigger breakdown delay time and switch jitter were very stable, and no pre-fire was found during about 2000 triggered shots. Therefore, we think the C4F7N/N2 gas mixture with a very little amount of C4F7N can dramatically improve the switch performance. It is valuable and easy to realize the application of C4F7N/N2 mixture in large scale pulsed power facilities.

Thinking on the Teaching Reform of Electronic Technology

with the rapid development of electronic technology, this paper starts with the design and manufacture of a kind of equipment in the reform of electronic technology teaching, and develops an electronic product. The reason is that the memory of the elderly is declining year by year, and people often forget to bring things when they go out of the house. For example, cell phones, door keys, etc., more serious is to forget to close the electric kettle in use, electromagnets, gas switches and so on. In view of the above phenomenon, I designed a warning device to go out, from shallow to deep in teaching, case teaching, to enhance studentsundefined interest in learning. The product can give a reminder when you go out, check to see if you have everything in place, and remind you to turn off the electrical appliances and gas switches that are in use. So that people can work in peace of mind, happy home.